Generator Size Calculator
What the Generator Size Calculator does
The Generator Size Calculator estimates the generator capacity needed for appliances and equipment you expect to run at the same time. It combines their running load with the largest additional starting surge, adds your chosen reserve margin, and displays the result in kilowatts (kW).
Generator sizing is about more than adding up appliance labels. Some equipment, especially appliances with electric motors, briefly needs more power when it starts. A refrigerator, well pump, or air conditioner may have a manageable running load but require a larger burst of power during startup.
This generator size calculator gives you a starting point for comparing available models. You should also check that a generator has enough continuous running capacity for your ongoing load and enough starting capacity for the anticipated surge. A single estimated kW figure does not guarantee that every appliance will work with a particular generator.
How to use the Generator Size Calculator
Gather the running and starting requirements for the equipment you want to power. Manufacturer documentation and appliance nameplates are better sources than generic wattage lists because actual requirements vary by model.
- Enter the total simultaneous running load in watts. Add the running watts of every appliance and device you expect to operate together. Include equipment that may cycle on automatically while the other loads are running.
- Enter the largest additional starting surge in watts. For an appliance that needs extra power to start, subtract its running watts from its total starting watts. Enter the largest additional surge you expect while the other loads are running.
- Choose a reserve margin as a percentage. The calculator applies this margin to the combined running load and additional surge. A margin provides room above your estimate, but it does not replace a check of the generator’s ratings.
- Review the Estimated Generator Size. The result appears in kW. Compare it with available generator sizes and round up to a suitable model that meets both running and starting requirements.
The calculator assumes that only one major motor starts at a time. If two or more motors can start simultaneously, include their combined additional surges in the starting surge input. The available input ranges are 0 to 100,000 watts for running load, 0 to 50,000 watts for additional starting surge, and 0% to 50% for the reserve margin.
How the Generator Size Calculator formula works
The calculator uses this formula:
Estimated Generator Size (kW) = (total running watts + additional starting surge watts) × (1 + reserve margin ÷ 100) ÷ 1,000
First, it adds the running watts of the simultaneous loads to the additional surge. Next, it applies the reserve percentage to that combined amount. Finally, it divides by 1,000 to convert watts to kilowatts.
Worked example: Suppose your simultaneous appliances use 3,000 watts while running. The largest additional starting surge is 2,000 watts, and you choose a 20% reserve margin. These are illustrative assumptions, matching the calculator’s default inputs.
- Running load plus additional surge: 3,000 + 2,000 = 5,000 watts
- Apply the 20% reserve: 5,000 × 1.20 = 6,000 watts
- Convert to kilowatts: 6,000 ÷ 1,000 = 6.00 kW
The displayed Estimated Generator Size is 6.00 kW. Use that figure to begin comparing generators, then verify the continuous rating against the running load and the starting rating against the expected startup demand. If an available model does not meet the relevant ratings, move up to one that does.
Use cases for the Generator Size Calculator
Planning essential home backup: List the appliances you want to operate during an outage, such as a refrigerator, selected lights, and communications equipment. Add only the loads you expect to use together, while accounting for appliances that can switch on automatically.
Comparing portable generators: Use the estimate to narrow your search before reviewing individual models. Two generators with similar advertised sizes may have different continuous and starting ratings, outlet configurations, or operating limitations.
Powering tools at a worksite: Add the running loads of tools and other equipment that will operate at the same time. Pay particular attention to motor-driven tools and whether more than one could start together.
Planning recreational or outdoor power: Estimate the combined load of the devices you intend to use simultaneously. If you plan to alternate equipment rather than run everything at once, make sure that operating plan is realistic and that users understand it.
Evaluating a backup power plan: Try different combinations of simultaneous loads to see how reducing or staggering usage changes the estimate. For example, postponing a large appliance until another load is off may reduce the capacity you need.
Other factors to consider when calculating Estimated Generator Size
Running and starting ratings: Generator manufacturers may list continuous and peak or starting output separately. Your ongoing load must fit within the applicable continuous rating, while the anticipated startup demand must fit within the generator’s starting capability. Check how long the manufacturer allows the generator to deliver its starting output.
Voltage and phase: A suitable kW estimate alone does not establish compatibility. Check that the generator supplies the voltage and phase required by your equipment and has appropriate connections for the intended loads.
Power factor and special loads: Some equipment has electrical characteristics or startup requirements that a simple wattage calculation does not capture. Manufacturer guidance is particularly useful for sensitive electronics, variable-speed equipment, and loads with demanding motor starts.
Altitude and temperature: A generator’s available output can fall under certain operating conditions. Check the manufacturer’s derating guidance for the altitude and temperature where you plan to use it.
Changing loads: Your estimate depends on what runs at the same time. Thermostats, pumps, and other automatic controls can start equipment without warning. Include realistic combinations of loads and simultaneous surges in your calculation.
Safe installation and operation: Follow the generator manufacturer’s instructions and applicable local requirements. When connecting a generator to household wiring, consult a qualified electrician about the appropriate transfer equipment. Operate fuel-burning generators outdoors, away from openings where exhaust could enter a building.
Generator Size Calculator FAQ
Do I enter total starting watts or only the extra surge?
Enter only the additional surge above running watts. For example, if an appliance runs at 800 watts and needs 2,000 watts to start, its additional surge is 1,200 watts. Its 800 running watts should already be included in the total simultaneous running load.
What if two appliances can start at the same time?
Add their additional starting surges together and enter the combined amount. The calculator’s usual assumption is that one major motor starts at a time, so a simultaneous start requires a larger surge input.
How much reserve margin should I use?
The appropriate margin depends on your loads, how accurately you know their requirements, and the generator’s operating conditions. The default 20% is an illustrative assumption, not a universal recommendation. Check the equipment and generator manufacturers’ guidance when deciding what margin is appropriate.
Is a 6.00 kW result enough for every 6 kW generator?
Not necessarily. A generator advertised as 6 kW may have a lower continuous rating or other limits that affect your equipment. Check its continuous and starting ratings, voltage, phase, and any relevant derating before choosing it.
Can I reduce the generator size by running fewer appliances at once?
Potentially. The running-load input reflects equipment operating simultaneously, so a practical plan to alternate loads can lower the estimate. Make sure the plan also accounts for automatic starts and that each individual appliance remains compatible with the generator.